Scientists have found that hydrogen peroxide H2O2 may spontaneously form on the surface of small water droplets, which is contrary to popular belief about the stability and chemical inertness of water. The discovery may become the basis for new methods of chemical synthesis, methods of purification and processing of food, the authors write in an article published in the Proceedings of the National Academy of Sciences.
Water is found everywhere on the Earth's surface, in the upper crust and atmosphere. Despite the fundamental role of this compound in any known form of life, many physical and chemical properties of water remain without a full theoretical explanation. For example, some ice features have led to the theory that water is a mixture of two different liquids. However, more careful further experiments have not confirmed this idea, as scientists have not been able to detect the predicted phase transitions.
The chemical stability of water is considered a well-established fact. However, some water molecules are constantly dissociating to ions, i.e. one molecule loses a proton, while the other attaches it, resulting in OH and H3O+ compounds, respectively. Under normal conditions, there is also another form of hydrogen and oxygen that is close to water: hydrogen peroxide H2O2, which is much less common due to instability.
Chemists from the United States and South Korea under the leadership of Richard Zare from Stanford University described the results of experiments that contradict the opinion on the chemical resistance of water. Scientists have found that small droplets of water without any external influences formed hydrogen peroxide. Moreover, the concentration of peroxide increased with the reduction of droplets and decreased with the addition of pure oxygen.
Researchers initially tried to improve the synthesis of gold nanoparticles in microdroplets of water but noticed abnormal results, which spoke about the new properties of water in this form. They began to conduct experiments with various combinations of parameters. The easiest option was to spray microdroplets between 1 and 20 microns onto an indicator strip whose color changes in the presence of peroxide.
It turned out that even pure water stains the strip when sprayed on small droplets. The authors confirmed the synthesis of the droplets by alternative methods, including the splitting (4-carboxyphenyl) of boric acid into boric acid and hydroxybenzoic acid, as well as the conversion of phenylboronic acid into phenols.
Scientists have also carried out experiments in which different gases are used for spraying: dry air, molecular nitrogen, and molecular oxygen. Air and nitrogen gave rise to approximately the same peroxide formation in a concentration of about 30 micromoles per liter (about one millionth), while oxygen atomization reduced the concentration.
The authors conclude that it was the water that gave rise to the additional oxygen, rather than dissolving it from the surrounding gas. The saturation of water with dissolved oxygen by pre-pulling the gas over time has also led to a decrease in peroxide concentration. Therefore, dissolved oxygen is also not involved.
Chemists offer several hypotheses explaining the appearance of peroxide: triboelectric effect, asymmetric separation of charges, contact electrification and oxidation of water due to surface electrical potential at the border with air. The authors name the last variant, in which peroxide is formed from two hydroxyl, as the most probable.
The results obtained in this paper can be useful both for basic research and in applied areas. In particular, such a process may be responsible for the formation of peroxide in rain droplets. If during the daytime peroxide formation in droplets is likely to be caused by photochemical reactions involving ozone, the peroxide observed in night rains may be due to the small size of some droplets.
In terms of technology, discovery can form the basis for new methods of obtaining peroxide, which is an important commercial and industrial reagent. The oxidizing properties of peroxide, which decomposes with the release of atomic oxygen, have been mainly used. Peroxide is used as a bleach, rocket fuel (both as an oxidizer and in a single component form), a catalyst and in the synthesis of many chemical compounds, including drugs.
The high reactivity of peroxide makes it possible to use it to make explosive mixtures, which we recently released a test. We talked to the British chemist Graham Hutchings about some of the options for using peroxide in an interview.